The preparation of cationic nitrogen-based surfactants involves the reaction of a tertiary amine with a quaternising agent in order to impart a positive charge to the nitrogen atom. This reaction can be carried out in a variety of solvents which may be aqueous or anhydrous, but a lower aliphatic alcohol-water mixture is normally employed commercially. Excess quaternising agent is removed from the reaction product by evaporation, after which the cationic surfactant may be purified in one or more work-up stages, to remove unreacted starting material or by-products and to improve product colour.
Nevertheless, separation and purification of the cationic surfactant is difficult and expensive, and, indeed, certain cationic surfactants form solids which cannot easily be handled in this way. This may be because the hydrophobic portions of the molecule contain a range of hydrocarbon chain lengths which may have different points of substitution or because the molecule contains groups such as hydroxy alkyl groups which are very difficult to produce as crystalline solids. This difficulty is compounded by the tenacity with which these materials retain solvents such as lower aliphatic alcohols and water so that the production of such cationic surfactants in solid form is unattractive commercially.
For this reason most cationic surfactants are offered commercially as solutions or dispersions in water or in a lower aliphatic alcohol-water mixture such as for example isopropanol-water, this being the solvent medium in which the quaternisation is carried out. This imposes certain formulation constraints where a solid cationic surfactant is required or where the presence of a volatile solvent is undesirable, e.g. in product whose physical form is not liquid and/or where the processing of such products would be adversely affected by the presence of a solvent.
It has now been found that this difficulty can be overcome by carrying out the preparation of cationic surfactants in an organic medium which is itself a component of the final product, but which is liquid under the conditions employed for quaternisation. One advantage of this procedure is that it permits the formation of the desired cationic surfactant as a finely divided dispersion, or in some cases a solution in the other product component, without the need to use solvents which require recovery or disposal. A further advantage is that it avoids the necessity of isolating and separately adding the cationic surfactant to the product, further simplifying its incorporation. Additionally, as described hereafter, the procedure offers an inexpensive and commercially attractive route to the manufacture of certain highly preferred cationic surfactant materials.